Taxols A and B are also known in the literature as taxol and cephalomannine and taxol C has no other name Mise en evidence nouveaux analogues du taxol extracts de Taxus baccata. V. Senilh, S. Blechert, M. Colin, D. Guenard, F. Picot, P. Potier and P. Varenne, Journal of Natural products 47, 131-137 (1984)!. Out of the three taxols A, B, C, only taxol A (=taxol) is used clinically for the treatments of ovarian and breast cancers. Various types of cancers have been treated with taxol A and the results in the treatments of ovarian and breast cancers are very promising. Taxol A (=taxol) has been approved by the Food and Drug Administration (FDA) of the United States for the treatments of ovarian and breast cancers.
Taxol B (=cephalomannine) has been isolated from the leaves, stems, roots of Taxus wallichiana; It is not clinically used. Antileukemic alkaloids from Taxus wallichiana. R. W. Miller, R. G. Powell, C. R. Smith, Jr., E. Arnold, and J. Clardy, Journal of Organic Chemistry 46, 1469-1474 (1981)!.
Taxol C has been isolated from the roots of Taxus media Taxol analogues from the roots of Taxus media. L. Barboni, P. Garibaldi, E. Torregiani, G. Appendino, B. Gabetta and E. Bombardelli, Phytochemistry 36, 987-990 (1994)!. Taxol C has also been isolated from the cell cultures of Taxus baccata and it showed potent and selective cytotoxicity against cell lines of non-small-cell lung cancer, small cell lung cancer, colon cancer, CNS cancer and ovarian cancer; New bioactive taxoids from cell cultures of Taxus baccata. W. Ma, G. L. Park, G. A. Gomez, M. H. Nieder, T. I. Adams, J. S. Aynsley, O. P. Sahai, R. J. Smith, R. W. Stahlhut, P. J. Hylands, F. Bitsch and C. Shackleton, Journal of Natural Products 57, 116-122 (1994)!.
Taxol A, a highly oxygenated diterpenoid molecule and a potent anticancer drug was first isolated from the stem bark of Taxus brevifolia. Thereafter, it has also been isolated from other Taxus species including Taxus wallichiana. Taxol A, a structurally complicated and chemically labile molecule needed special and careful extraction and separation procedures for its isolation from plant materials. Unfortunately, most of the works are proprietary in nature and have not been published. The American workers have used alcohol to extract taxol from the stem bark of T. brevifolia and isolation of taxol from the alcoholic extract used sequential column chromatography over silica with methanol-chloroform mixture (2: 98) as the eluting solvent to yield a mixture of taxol A (=taxol) and Taxol B (cephalomannine). In one of the prior art process, Taxol A has been separated and isolated from the mixture containing taxol A and Taxol B with a yield of 0.01% either by repeated column chromatography over silica gel or by high performance liquid chromatography (HPLC). M. C. Wani, H. I. Taylor, M. E. Wall, P. Coggan and A. T. Mc Phail. Plant antitumor agents VI: The isolation and structure of taxol, a novel antileukemic and antitumor agent from Taxus brevifolia. Journal of the American Chemical Society 93, 2325 (1971); and J. H. Cardellina: HPLC separation of taxol and cephalomannine, Journal of liquid chromatography 14, 659 (1991)!.
According to another prior art process, Taxol A has been isolated from the stem bark of Taxus wallichiana with a yield of 0.02%. The isolation process involves extracting the stem bark with methanol, partitioning of the methanolic extract between water and chloroform and isolation of taxol A from the chloroform soluble fraction by column chromatography over silica gel. S. K. Chattopadhyay, V. K. Tripathi, R. S. Thakur, R. P. Sharma and S. P. Jain. Isolation of taxol, 10-deacetyl baccatin III and (-) betuligenol from Taxus baccata 33B, 409 (1994)!.
According to a prior art process, taxol analogues 7-xylosyl-10-deacetyl taxol A and B have been converted into taxol A (=taxol) and taxol B (cephalomannine) by K. V. Rao Process for the preparation of taxol and deacetyl taxol, K. V. Rao, U.S. patent application Ser. No. 851469, 13 Mar. 1992!.
The process for the preparation of taxols A or B involved reacting the analogue 7-xylosyl-10-deacetyl taxol A or B with periodate in methanol. Chloroform and sulphuric acid mixture at 20.degree.-60.degree. C. to give a dialdehyde product which was then treated with phenyl hydrazine in methanol-aqueous acetic acid mixture and heated at 50.degree.-60.degree. C. to degrade the dialdehyde into 10-deacetyl taxol A or B which was then isolated by column chromatography. The resultant 10-deacetyl taxol A or 10-deacetyl taxol B was then converted into taxol A (=taxol) or taxol B by reacting 10-deacetyl taxol A or B with an acetylating agent at 0.degree.-100.degree. C. to block the 2', and 7-hydroxyl groups, then acetylating the 10-hydroxyl group with an acetylating agent at 0.degree.-100.degree. C. followed by deprotecting 2' and 7-hydroxyl groups with a suitable deprotecting agent to taxol A or B.
The process for preparation of the intermediate product 10-deacetyl taxol A or B from 7-xylosyl-10-deacetyl taxol A or B, and conversion of 10-deacetyl taxol A into Taxol A (=taxol) as described by Rao suffers from major disadvantages which include low yield of the intermediate product 10-deacetyl taxol A, or B, eg. 0.5 g 7-xylosyl-10-deacetyl taxols A, or B yields 0.2 g 10-daecetyl taxol A or B; as well as low yield of the final product taxol A (=taxol) from its intermediates 10- deacetyl taxol A; eg. 0.5 g 10-deacetyl taxol A yields 0.3 g taxol A. No experimental procedure and yield are reported by Rao for conversion of the 10-deacetyl taxol B into taxol B (=cephalomannine).
The reasons for low yield of the intermediate products 10-deacetyl taxol A or B may be rationalized due to (i) carrying out the periodate oxidation of the 7-xylosyl-10-deacetyl taxol A or B in presence of mineral acid (although it is mentioned by Rao in his patent application that periodate oxidation can also be carried out under neutral condition in presence of excess sodium bicarbonate, the presence of excess dissolved sodium carbonate may lead to degradation of the side chain from dialdehyde leading to mixture of products) (ii) heating the periodate oxidation product with phenyl hydrazine in methanol-aqueous acetic acid mixture at 50.degree.-60.degree. C. to degrade the periodate oxidation product into 10-deacetyl taxol A or B.
The reasons for low yield of the final product taxol A (=taxol) from the intermediate product as described in Rao's process may be due to selection of improper protecting groups to block 2', 7-hydroxyl groups and non-optimization of reaction conditions such as duration of reaction time and temperature to carryout the protection of the groups.
Moreover, the processes for production of the intermediate product 10-deacetyl taxol C from the analogue 7-xylosyl-10-deacetyl taxol C and conversion of the above said intermediate product 10-deacetyl taxol C into taxol C have not been covered by Rao in his patent application.